This invention relates to a method of welding seam portions of a pipe fashioned from a steel plate.
When welding along a seam line of a pipe fashioned from a steel plate, the heat affected zone (hereinafter abbreviated occasionally to HAZ) will become brittle due to marked formation, in the course of the welding heat cycle, of an upper bainite in the vicinity of a bond or transition zone between the weld metal and parent metal.
Such brittleness of the heat affected zone is undesirable especially when a low temperature toughness in excess of a predetermined value is a requirement. It is believed that high toughness of the weld metal may not be realized due to formation of the proeutectoid ferrite. An accepted practice is to cool the heat affected zone during welding. In the case of submerged arc welding, hereinafter abbreviated occasionally as SAW, the cooling of the weld or weld joint may be caused by spontaneous cooling of the weld zone surrounded by the fluxes, dissipation of heat from the lower surface of the parent metal and the transfer of heat to the parent metal. Reliance on such spontaneous cooling solely gives rise to retarded cooling of the weld surrounded by a thick layer of slags, thus the Charpy strength of the weld being lowered. Such tendency will be more noteworthy when the heat input is elevated. For this reason, a variety of cooling methods have been proposed for cooling the weld, including water cooling used in conjunction with vertical electro-slag welding. This method is however not applied to the case of submerged arc welding. It is also known to cool the weld surface with a mist after welding and subsequent to the flux removal. With such cooling, the start time of cooling may be delayed due to hygrospic properties of the fluxes and the mounting of the flux recovery device between the cooling position and the electrodes. Moreover, such cooling technique can not be used for cooling the inside weld of a large diameter pipe which has been welded on the outside by the preceding welding operation.
In submerged arc welding, endeavors have been made to improve the toughness of the heat affected zone as by using fluxes of higher basicity or adding to the weld metals alloying agents such as nickel, molybdenum and titanium. It was also proposed to perform a so-called multi-layer welding at the sacrifice of welding efficiency or to add the parent metal with trace amounts of Ti, Al-B, REM-B Ti-Ca or similar combinations of alloying agents. Especially, higher notch toughness is required of the weld metal and the heat affected zone of large diameter pipes that are designed for the frigid zone such as Arctic Zone where the temperatures may fall to -25.degree. to -40.degree. C. It is a matter of great difficulty to satisfy the above requirement by using the currently marketed steel plates and welding wires at an acceptable cost.